Studies of the innate and the adaptive immune responses generally follow different paths. Scholars of adaptive immunity focus on the inductive phase of immune response as na'fve animals have extremely low frequencies of antigen-specific lymphocytes, and lymphocytes must be expanded substantially to exert meaningful immune protection. In contrast, because the innate immune system does not use clonally distributed receptors, a substantial number of effector cells must be available for protection against infected or malignant cells. The study of innate immunity, particularly that mediated by natural killer (NK) cells, has been focused on regulating the function of pre-existing effector cells. However, recent studies suggest that the two branches of immunity may share more features. For example, the receptors involved in the regulation of NK cells are also found on activated T cells, and molecules that inhibit NK function also inhibit T cell activation. An intriguing possibility is whether the reverse is also true. That is, whether the principles learned from T cell activation can be applicable to NK cell biology. An important principle in T cell activation is that optimal T cell responses involves both specific antigen and costimulatory molecules. Our preliminary data suggests that two signals may also be involved in the NK cell function. The major focus of this proposal is whether the two signal theory of T cell activation also governs the behavior of NK cells. Specifically, we propose to investigate whether NK cell activation in the setting of an in vivo anti-tumor response requires costimulation with both an activating NK cell receptor ligand and known costimulatorv molecules such as the B7 family members. Furthermore, we will systematically evaluate the role of CD28 and CTLA4 in B7-dependent NK cell responses determining whether these receptors, or other unknown B7-receptors, are operative during NK anti-tumor responses. Our proposed studies, in combination with other projects in this Program, may lead to novel approaches to enhance innate immunity for anti-tumor therapy.